Facsimile transmitting and reproducing system, method, and apparatus



2,403,358 FACSIMILE TRANSMITTING AND REPRODUCING-SYSTEM, METHOD, AND APPARATUS J ly 2, 1946- A. E. -GERHARD ETAL 1 M w t W m r E m w m WNW S N T 5% A 2 HQRGS. W Y f a w \W 6 1 HNG-Q YWR r 031w m mt F m R kw n o July 2, 1946. A. E. GERHARD ETAL 2,403,353 FACSIMILE TRANSMITTING AND REPRODUCING SYSTEM, METHOD, AND APPARATUS 2 Sheets-Sheet 2 Original Filed April 16, 1941 rlll l t I I I '0 l I I I l 0 0 I l t I i I 0 l I v M Armwra s IIOICIQIlIIl Patented July 2, 1946 DUCING SYSTEM, ME

TUS

THOD, AND APPARA- Anthony E. Gerhard, Los Angeles, Calif., and Everett G. Fraim, Hicksville, N. Y., assignors to Press Wireless, Inc., Chicago, 111., a corporation of Delaware Original application April 16, 1941, Serial No.

Divided and this application April 30,

1942, Serial No. 441,120

9 Claims.

A feature of the invention relates to an improved system for converting picture shades and the like into a frequency-modulated carrier the spectrum of which is confined to the audio frequency range.

Another feature relates to animproved frequency-modulating arrangement whereby the signal amplitude variations corresponding to picture shades are converted into a frequencymodulated audio frequency carrier by a novel 0 combination of oscillator tubes and a control tube therefor.

A furtherfeature relates to an improved manner of varying the frequency of an oscillator tube by varying the impedance of a grid-controlled tube whose anode-cathode discharge path is effectively in shunt to an oscillatory circuit through a condenser of low capacity, whereby the variations of impedance of the control tube are reflected as variations in shunt capacitance across said oscillatory circuit.

Another feature relates to an improved converter for translating the output of a facsimile transmitting machine of the type producing picture signals in the form of an amplitude-modulated audio frequency carrier, into a frequencymodulated carrier limited to the audio frequency range.

Another feature relates to a receiver for facsimile systems wherein the subject matter is transmitted as a frequency-modulated audio frequency carrier, the receiver having means to convert the frequency-modulated carrier into corresponding amplitude modulations while limiting the conversion to a frequency spectrum wherein the uppermost frequency is less than the second harmonic of the lowermost frequency.

Another feature relates to an improved converter for translating a received frequencymodulated audio frequency carrier into corresponding variable-amplitude D. C. signals.

A still further feature relates to the novel organization, arrangement and relative interconnection of parts whichconstitute an improved facsimile transmission system.

I 2 Other features and advantages not specifically enumerated will be apparent after a consideration of the following detailed descriptions and the appended claims.

Referring to the drawings which by way of example shows one preferred embodiment,

Fig. 1 is a schematic wiring diagram of the transmission end of a facsimile system embodying features of the invention.

Fig. 2 is a schematic wiring diagram of the receiving end of a facsimile system embodyin features of the invention.

Referring to Fig. 1, the numeral I represents diagrammatically any well-known form of facsimile transmitting machine whereby the shades or tone values of successive elemental areas of the subject matter are scanned and translated into a corresponding amplitude-modulated audio frequency carrier, for example a carrier of 1800 C. P. S. Reference may be had to U. S. Patent No. 2,015,742'for a typical machine of this type. The output of machine I is applied through transformer 2 across the potentiometer resistance 3, the adjustable arm 4 of which is connected to grid 5 of amplifier tube 6. The potentiometer is also connected through condenser I to the cathode 8. Grid 5 is negatively biassed with respect to the cathode by the IR drop caused by the plate current flow through resistor 9; and a suitable series resistance I0 is connected between potentiometer 3 and ground. The steady high voltage potential for plate I I is applied over conductor I2, series resistor I3 and transformer primary I4. Conductor I2 is supplied with high voltage D, C. from any well-known source consisting for example of the A. C. ma ns I la; stepup transformer I5; full wave rectifier I6; filter I'I, I8, I9, 20 and any well-known voltage stabilizer 2|.

The amplitude-modulated A. 0. wave from transformer I4 is applied to a frequency doubling arrangement comprising a, twin triode tube 22,

the input circuits between the grids 23, 24, and cathodes 25, 26, being connected in balanced relation while the plates are connected in phase to the primary winding of transformer 21. The output of tube 22 therefore consists of a 3600 C. P. S. wave which is amplitude-modulated in accordance with the original picture signals from machine I. This wave is then impressed upon a full wave rectifier tube 28 in the output circuit of which is connected a, suitable filter comprising elements 29, 30, 3|, 32, whereby substantially all frequency flutter is eliminated and there is deweloped acrossthe potentiometer resistance 31,

a D. C. voltage of varying amplitude corresponding to the shades of the original subject matter scanned by the machine l.

The rectified signal from tube 28 is applied, to control a frequency modulator comprising a variable-frequency oscillator tube 33 and a fixed fre- 4 55 to the the first control grid 51 of tube 54, the said grid being normally negatively biassed by cathode resistor 53; a suitable leak resistor 59 be ing provided. Likewise, the .fixed frequency voltage developed across resistor 60 is applied through coupling condenser 6| to the second conquency oscillator tube 34 and a control tube 35;

The tube 35 may be of the triode type whose control grid 36 is connected to the adjustable arm of potentiometer 31 through a suitable bias battery 38 whereby grid 36 is negatively biassed with respect to cathode 39. Plate 40 is connected to ground, as is the low potential-end of potentiometerSl. The grid-cathode return circuit of tube 35 is completed through the control-grid to cathode trol grid 62. A suitable steady positive potential is applied to the .anode-grid 63 by means of conductor 64." The fourth grid 65 is connected directly to the grid 51. By the well-known action of this type of tube, there is developed across "theplate load resistor 65 voltages whose freenemy is. determined by the difierence frequency circuit of tube 33 and includes the grid :leak re-.

sistor 4| and the lower section of oscillator coil 42. Coil t? is shunted by an adjustable tuning condenser 43 and this combination of coil and condenser is connected to the control grid 44,

through avery. low capacityrcondenserfiii, for example of about- .0001 mid. It willbe-seen therefore that the oscillatory. circuit. formedby 42 and 43 is efiectively shunted by acircuitconsisting of'the' condenser 45, resistance 41. and the cathode-to-plate discharge path. of tube. 55.

- Consequently; the frequencyofpscillation of tube -33 is determined in partby theelementsiflflgfiB,

as well as by theefiective shunt capacitance of a a condenser which in turn is controlled bywtube 35. It will be noted'that whiletheplate fltl of the control tube 35 is connected to: ground, it issefiectively at positive potentialewithrespect to the cathode 39 when the tube 33 is in an oscillating state since in that condition a grid-current flow exists between the grid 43 and the cathode 4B is of such a polarityas torenderthe grid fi l and consequently the cathode 39 negative with respect to ground. This is a D. C.-potentialdifierence because of the rectifying efiect between thecathode 36 and grid l l.

Tube 33' is a so-calledelectron-coupled oscillator; for'example a tube of the-type--'6J'7 comprising a cathode 46, control-gridd l. shield grid 41-, suppressor grid 58 and plate- 495 The upper section of coil-431s connected between and 46 iwhile'the lower sectionofcoil i'2 is" connected between liiand l'l, thus generatingoscillations by feed-back action. The elements 'MMS and ll,

form in'efiect a triode oscillator of which the element" 41 is in' effect the anode, the plate-49 acting as a collector electrode for the electrons'passed by grid 41.; The oscillator 34 is similar to oscil- "lator 33' except that thefeed -back coil50- is shunted by aflxed condenser-5| and the'control grid 52 is returned directly to "ground-through the leak'resistor'53: In both oscillators the respective suppressor grids may be connected directly to the associated cathodes. The controlled "oscillator 33 is designed'and set so thatvariations of frequency can be effected over arange of 1000 cycles corresponding respectively to the range'of tone values'or shades in the-"subject .matter being scanned by the machine Thus oscillator '33 by-means of potentiometer Iii-and ."bias battery 38 can be adjusted so that for "white .sistor55 are applied through coupling condenser.

between .the..-..oscillators 33 and 34. In other words, under the above assumed conditions, there is developed an audio frequency band of from 3500 to 4500 C. P. S. This variable frequency si nal is applied to the output transformer 51 through a radio frequency choke coil 38 and coupling condenser 69 of the order of .005 mfd. The

.. audio frequencyurnodulations .may..-then-be applied to control theamplitude modulation of any well-known formoftradio transmitter represent- .ed schematically by. the numeral 70.. It'will-be observed therefore, that the signals representing the shade valuesare in the form ofaudio frequency: mo dulations: having a frequency spectrum between 3500;and .4500 C. 39.5., so that .theuppermost signal frequency,. -namely;@500.C. E. S. is less than the second harmonic: oiithe lowermost signalfrequency, .namely,;3500 C. P. S.

At the receiving station, the received radio wave is detected and amplified by any well-known form'of radioreceiver H whereby the 3500 4500 audio frequency spectrum is applied tothe'coupling -transformer 12.: :Part ofrtheioutput of transformer 12 is fed to a volume-controlled amplifier tube13', for eXam-ple of :the typefiL'l ;.;and apart oftheoutput of transformer 12 is applied to the volume control or regulator-tube M.- which may-be of the type. 6R7. Forthe purpose ofcon trolling the output levels, the firstucontrclrgrid .215 -of.tube..13 is connected-to auva-riablemap l6 on the potentiometer. resistance 1 1.1;.Likewise, the control grid I8 of theregulatortube .14 is connected toanjadiustable.tap'19w TheJtube 'M is of :the: delayed action type. and doesnot start-zfuncxtiom'ng: until a. predetermined :.minimuml.- input 4 level, e. g.- .1'-2ydb. is reached; then the-regulator .tube'l l .takescontrol and...holds:.the outputoi tube 13 constant to within. approximately 0.5: db.

until a levelr'of 2 db. reachedcorresponding to the: beginning" of: overloading. :1 of the. amplifier.

The steady; potentials; for; the. plate: :80 .and" for the .plate 3=lrare gapplied *ovenconductortBZ: .trom the high voltageD. C.-;-supply:.unit which maybe :similar: to that-used atthe1transmitter. comprising A. C. mains 83; step-up transformer.:84;..full

stabilizer:;90.'. In accordance with'ithe well-known operation. of .the type.-.6R7..tube, that is..tube .14,

asithe platencurrentiincreases it applies BUCDI'I'E- controlgrid; 93 of. tubefl3.

tion of resistors 94, 95 and condenser-'95, are. prosponding i. increased potential; :to :3 the auxiliary anodes 9| whereby a corresponding-massing.potential; is'developed across;..the;.cathode resistor 92 thus controlling. .the current-r. toe. plate 30.; and therefore determining the potential. orthasecond A. :suitable. combinavidedhaving a predeterminedi:time constant to determine the extent ofdelayoi'action ofthe-regulator. tube 14 on gridg93.

The plate or; output circuit of; tube 13- isconnected. through a.-resonant:;circuit consisting. of the series condensers. 91, 98;:resistor39gaind aparallel winding I of the couplingtransformer IOI and shunt condenser I03. The elements of the combination 0'! to I03 are chosen in magnitude so as to provide a resonant circuit which is resonant at 3500 C. P. S. and which has a sloping characteristic on either side of 3500 cycles with a maximum attenuation at 2500 and 4500 cycles respectively. Consequently the signals that are applied to the control grid I04 of a succeeding amplifier tube I05 are confined to the range between 2500 and 4500 C. P. S. Consequently,- if theriehas been any harmonic distortion in the form of frequency doubling. at any point between the transmitter I0 and the receiver II, it is effectively eliminated since the second harmonic of the lowermost signal frequency of 3500 C. P. S. is not passed to the amplifier tube I05. Tube I05 may be of the type 6N7 for amplifying the 2500- 450.0 spectrum applied to the control grid thereof. The output of tube I05 is then passed through a frequency doubling tube I06 of the type 6N7 similar to the tube 22 at the transmitter. The doubled frequency signals are then applied to the full wave rectifier tube I01, the circuit of which includes a suitable filter comprising elements I08, I09, IIO, III, H3, H4, H5, H0, whereby substantially all frequency flutter is eliminated and the voltage developed across the potentiometer resister I I1 consists of D. C. signals with amplitude variations substantially identical with the amplitude variations from the tube 28 at the transmitter. These D. C. amplitude variations which represent the original picture shades at the transmitter machine I, can be used to control the reproducing element of any well-known form of facsimile reproducing machine represented by the numeral II8 which is maintained in time and phase synchronism with the transmitting machine I in any manner well-known in the facsimile art. If the machine I I8 is of a type which is controlled by a modulated audio frequency wave, then it is necessary to convert the variable amplitude D. C. signals across resistor II'I into a corresponding amplitude-modulated audio frequency carrier. For this purpose, there is provided an audio frequency oscillator tube I I9 which may be of the type 6J7 having a feed-back oscillator coil I20 connected between the control grid I2I and the cathode I22, another section of this coil being connected between the cathode I22 and the anode-grid I23. The suppressor grid I24 may be connected directly to the cathode. A tuning condenser I250, is connected across the oscillatory circuit to control the oscillation frequency, for example 1800 C. P. S. The operating potentials for the anode-grid" I23 and the collector plate I26a are derived over conductor 82. The 1800 cycle signal from oscillator H9 is applied in push-pull relation to the input grids I25 of the mixer tubes I26, I21, which may be of the type 6L7. The D. C. signals representing the picture shades are tapped off from the potentiometer I I1 and are applied in parallel to the second control grids I28 of tubes I26, I21. The plates of the tubes I26 and I21 are connected in balanced relation through the output resistor I29, the steady plate potentials being derived over conductor I30. Consequently there is developed across resistor I20 an 1800 cycle signal having amplitude modulations corresponding to the original picture shades. This amplitude modulated wave is then passed through a suitable amplifier I3I which may be of the type 6N7 preferably connected to operate as a class A output amplifier to raise the signal to the desired level for controlling the reproducing element of thefacsimile machine II8.

I have found that with the foregoing arrangement, a satisfactory picture having a wide range of tone values can be transmit-ted over long distances either by wire or by radio and the effects of fading and harmonic distortion such as frequency doubling and the like in the transmitting link, are substantially eliminated. While specific apparatus and circuits have been disclosed for achieving the objects of the invention, it will be understood that various changes and modifications may be made therein without departing from the spirit and scope of the invention.

This application is a division of application Serial No. 388,742, filed April 16, 1941.

What is claimed is:

1. A frequency modulating system comprising, a source of signals of variable amplitude and means to convert said amplitude variations into corresponding frequency variations for signal transmission and representing the signals from said source including a tuned inductance and a variable shunt circuit across said tuned inductance, said variable shunt circuit including in series a low capacity condenser and a grid-controlled electron tube with its anode at substantially ground potential; and means to impress said variable amplitude signals between the grid and anode of said tube varying said shunt and thereby to control said conversion.

2. A frequency modulating system comprising, an oscillator tube for generating variable frequencies said tube having a D. C. control gridcathode return circuit, a, control tube for varying the frequency of said oscillator tube, said control tube having an anode, a cathode and a control grid, said anode being grounded, and a D. C, return path for the anode-cathode said control tube including the control grid to cathode D. C. return circuit of said oscillator tube, and means to vary the conductivity of said control tube by variable amplitude signals applied between the control grid and anode of said control tube to thereby produce frequency-modulated waves representing original signals to be transmitted.

3. A frequency modulating system comprising, an oscillator tube for generating variable frequencies, a control tube for varying the frequency of said oscillator tube the plate of said control tube being substantially at ground potential, a D. C. circuit to bias the control grid of said oscillator tube with respect to its cathode, said circuit including the D. C. return circuit of the anode-cathode discharge space of said control tube, and means to vary the conductivity of said control tube by variable amplitude signals and thereby to vary said bias applied between the control grid and anode of said control tube and to cause said oscillator tube to produce desired frequency-modulated waves representing the original signals to be transmitted.

4. A system according to claim 3 in which said control tube has its anode substantially directly grounded and the potential for the space disguests means connecting-said control t'ube'to said'o se il lator tube so that the potential for producing the" space discharge between the cathode and anode of said control tube is derived from the grid potential of said oscillator tube;"and means to impress D, C. signals between the grid and anode of said control tube: to vary thereby" the" frequency of said oscillations; I

6. A frequency modulating system comprising an-oscillator-generator tube, atunable oscilla tory circuit connected to said tube, and anothercirc'uitconnecting- 'thecontrol grid of said--osc'i1- lator tube to ground said other circuit including a, grid-controlled electron tube' havingits cathode connected to the grid of said oscillator tube and its plate substantially at ground potential', the* grid of the last-mentioned tube being" connected toa's'our'ce' of D. C. signal potentials for varying correspondingly the potential between the gridana anode of said electron tube and thereby" varying the frequency of said oscillations to rep; resent the original signals to be transmitted, and a utilization circuit to which saidvaried ire quency oscillations are applied;

7. A frequency-modulator arrangement for .4

converting amplitude modulations into irequem' cy modulationsfor transmission and represent ing original signalsto betrans'mitted compris ingan oscillator g'enerator tube having a circuitconnected thereto for generating oscillations byfeed-back actionto a control grid of' said tube,- a variable resistance tubehaving a cathode,- an anode at groundpotential and controlgrid; means-connectingthe D. C; return oircuitof the anode-cathode space discharge'path of' said vari-' able resistance tube in common withthe D. 0. return circuit of theoscillator-generator tube s'o' that the rectified grid-current of'the oscillator tube flows therethrough; and meansto impressamplitude-modulated signals between thecontrol grid and anode ofsaid ariable resistance tube to Vary thereby the frequency of the oscillations fromsa'id generator tube, anda utilization circult to which the varied frequency oscillations are applied. I V 1 8. In' an electric wave signalling system of the frequency modulation type, wherein the' signals aretransmitted over a, communication channel which introduces-distortion such 'as' frequency doubling and wherein the frequency modulated signals areli'mited to an audio frequency spectrum wherein the uppermost frequency is lessf than the second harmonic of" thelowermost fre quency, means to receive andreproduce' said'sig 1 n'als, comprising a grid-controlled amplifier tube;

a grid-controlled regulator tube therefon'means to impress a,portionofthereceiv'ed signals'simuk taneously' on the control grids of both tubes;- means normally biassing the-control grid of the regulator tubefl so that it exerts substantially negligible control on said amplifier tube until the incoming signal level is above a predetermined value, said amplifiertubehaving' a level control grid, means to impress on said level control grid a-D'C. potential controlled by theplate current of said regulator tube, and anetwork in the output of said amplifier tube which is resonant at a frequency approximately' midway of said spectrum and" which has maximum at tenuation at the upper andlower endsof said' spectrum; I

9. A system according to claim 8' wherein said as to'produce modulations therein under control of saidrectifier;

ANTHONY E. GERI IARD; EVERETT G'. FRAIM. 

